U.S. patent application number 12/923302 was filed with the patent office on 2011-03-17 for ultrasonic transducer, ultrasonic probe and producing method.
This patent application is currently assigned to FUJIFILM Corporation. Invention is credited to Takatsugu Wada.
Application Number | 20110062824 12/923302 |
Document ID | / |
Family ID | 43416841 |
Filed Date | 2011-03-17 |
United States Patent
Application |
20110062824 |
Kind Code |
A1 |
Wada; Takatsugu |
March 17, 2011 |
Ultrasonic transducer, ultrasonic probe and producing method
Abstract
An ultrasonic probe having an ultrasonic transducer is provided.
The ultrasonic transducer includes a first piezoelectric layer for
transmitting ultrasonic waves to an object in a body. A second
piezoelectric layer receives the ultrasonic waves reflected by the
object. An acoustic matching layer is disposed between the first
and second piezoelectric layers, for constituting an electrode
common to the first and second piezoelectric layers. Preferably,
the second piezoelectric layer is positioned nearer to the object
than the first piezoelectric layer. The first piezoelectric layer
is inorganic, and the second piezoelectric layer is organic. The
acoustic matching layer contains metal, for example, silver. In one
embodiment, the second piezoelectric layer includes plural
receiving element regions, arranged in a first direction, and
having volumes different from one another.
Inventors: |
Wada; Takatsugu; (Kanagawa,
JP) |
Assignee: |
FUJIFILM Corporation
Tokyo
JP
|
Family ID: |
43416841 |
Appl. No.: |
12/923302 |
Filed: |
September 14, 2010 |
Current U.S.
Class: |
310/334 ;
29/25.35 |
Current CPC
Class: |
Y10T 29/42 20150115;
B06B 1/0629 20130101; A61B 8/4427 20130101; B06B 1/0622 20130101;
A61B 8/4444 20130101; B06B 1/064 20130101; G10K 11/02 20130101 |
Class at
Publication: |
310/334 ;
29/25.35 |
International
Class: |
B06B 1/06 20060101
B06B001/06; H01L 41/26 20060101 H01L041/26 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 15, 2009 |
JP |
2009-212663 |
Sep 28, 2009 |
JP |
2009-222007 |
Sep 29, 2009 |
JP |
2009-223691 |
Claims
1. An ultrasonic transducer comprising: a first piezoelectric layer
for transmitting ultrasonic waves to an object in a body; a second
piezoelectric layer for receiving said ultrasonic waves reflected
by said object; and an acoustic matching layer, disposed between
said first and second piezoelectric layers, for constituting an
electrode common to said first and second piezoelectric layers.
2. An ultrasonic transducer as defined in claim 1, wherein said
second piezoelectric layer is positioned nearer to said object than
said first piezoelectric layer.
3. An ultrasonic transducer as defined in claim 2, wherein said
first piezoelectric layer is inorganic, and said second
piezoelectric layer is organic.
4. An ultrasonic transducer as defined in claim 2, wherein said
first piezoelectric layer is inorganic, and said second
piezoelectric layer is a complex containing an organic compound and
an inorganic compound dispersed in said organic compound.
5. An ultrasonic transducer as defined in claim 1, wherein said
acoustic matching layer contains metal.
6. An ultrasonic transducer as defined in claim 5; wherein said
metal is constituted by silver.
7. An ultrasonic transducer as defined in claim 1, wherein said
acoustic matching layer is a complex containing an organic compound
and metal.
8. An ultrasonic transducer as defined in claim 7, wherein said
organic compound is adhesive.
9. An ultrasonic transducer as defined in claim 7, wherein said
metal is constituted by metal nano particles.
10. An ultrasonic transducer as defined in claim 1, wherein said
acoustic matching layer includes: a first layer of a material with
a relatively low conductivity or a non-conductive material; and a
conductive material overlaid on said first layer.
11. An ultrasonic transducer as defined in claim 1, further
comprising a second acoustic matching layer overlaid on a surface
of said second piezoelectric layer opposite to said acoustic
matching layer.
12. An ultrasonic transducer as defined in claim 1, wherein each of
said first and second piezoelectric layers includes plural element
regions arranged in an array in said first direction.
13. An ultrasonic transducer as defined in claim 1, wherein said
second piezoelectric layer includes plural receiving element
regions, arranged in a first direction, and having volumes
different from one another.
14. An ultrasonic transducer as defined in claim 13, wherein said
plural element regions have receiving surfaces, which are arranged
in said first direction, fox receiving said reflected ultrasonic
waves, and of which areas are different from one another.
15. An ultrasonic transducer as defined in claim 13, wherein said
plural element regions have thicknesses different from one another
in a direction perpendicular to receiving surfaces thereof.
16. An ultrasonic transducer as defined in claim 13, wherein said
element regions are arranged two-dimensionally in said first
direction and a second direction.
17. An ultrasonic probe having an ultrasonic transducer comprising:
said ultrasonic transducer including: a first piezoelectric layer
for transmitting ultrasonic waves to an object in a body; a second
piezoelectric layer for receiving said ultrasonic waves reflected
by said object; and an acoustic matching layer, disposed between
said first and second piezoelectric layers, for constituting an
electrode common to said first and second piezoelectric layers.
18. An ultrasonic transducer comprising: a piezoelectric layer;
plural acoustic matching layers overlaid on said piezoelectric
layer; said acoustic matching layers including a first acoustic
matching layer, positioned on said piezoelectric layer, formed from
a mixture of metal nano particles and adhesive resin, for
constituting an electrode for said piezoelectric layer.
19. An ultrasonic transducer as defined in claim 18, wherein said
metal nano particles are constituted by silver nano particles.
20. An ultrasonic transducer as defined in claim 18, wherein said
acoustic matching layers are at least three acoustic matching
layers of which acoustic impedance is different from one
another.
21. An ultrasonic probe having an ultrasonic transducer comprising:
said ultrasonic transducer including: a piezoelectric layer; plural
acoustic matching layers overlaid on said piezoelectric layer; said
acoustic matching layers including a first acoustic matching layer,
positioned on said piezoelectric layer, formed from a mixture of
metal nano particles and adhesive resin, for constituting an
electrode for said piezoelectric layer.
22. A method of producing an ultrasonic transducer having a
piezoelectric layer, comprising steps of: placing a first acoustic
matching layer on said piezoelectric layer, said first acoustic
matching layer containing a mixture of metal nano particles and
adhesive resin, and constituting an electrode for said
piezoelectric layer; placing a second acoustic matching layer
different from said first acoustic matching layer on said first
acoustic matching layer; hardening said first acoustic matching
layer by heating, so as to attach said first acoustic matching
layer to said piezoelectric layer and to attach said second
acoustic matching layer to said first acoustic matching layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an ultrasonic transducer,
ultrasonic probe and producing method. More particularly, the
present invention relates to an ultrasonic transducer of which
sensitivity can be raised with a simple structure, and ultrasonic
probe and producing method.
[0003] 2. Description Related to the Prior Art
[0004] An ultrasonic probe is widely used for medical diagnosis. An
ultrasonic transducer is disposed in a distal tip of the ultrasonic
probe. The ultrasonic transducer is constituted by backing
material, a piezoelectric layer, electrodes, acoustic matching
layers, and an acoustic lens. The ultrasonic transducer emits
ultrasonic waves to a human body, and receives reflected ultrasonic
waves or echo from the human body. An ultrasonic imaging device is
supplied with echo information from the ultrasonic transducer,
processes the same electrically to produce an ultrasonic image.
[0005] It is also possible to produce ultrasonic tomographic images
by emission of the ultrasonic waves in a scanned manner. Various
methods of producing the ultrasonic tomographic images are known.
In a method of mechanical scan, the ultrasonic transducer is
mechanically rotated, swung or slid. In a method of electronic
scan, a plurality of the ultrasonic transducers are arranged in an
array. One or more of the ultrasonic transducers to be driven are
changed over by an electronic switch or the like.
[0006] It has been very usual to use one piezoelectric layer to
transmit and receive the ultrasonic waves. There is another
structure in which a first one of the piezoelectric layers
transmits the ultrasonic waves and a second one of the
piezoelectric layers receives the reflected ultrasonic waves. For
the first one, an inorganic piezoelectric ceramic material is
suitably used with a relatively high value of an electromechanical
coupling coefficient k33 for high power in transmitting the
ultrasonic waves. For a second one, a polymeric piezoelectric
compound is suitably used with a small value of a transmission
coefficient d33 but with a relatively high value of a reception
coefficient g33 for good sensitivity for receiving the reflected
ultrasonic waves.
[0007] In JP-A 6-148154, one of the piezoelectric layers for
transmission is disposed on a proximal surface of the acoustic
lens, on a side opposite to the human body. The remainder of the
piezoelectric layers for reception are disposed in recesses formed
in the acoustic lens, on a distal side near to the human body.
[0008] There is a large difference in acoustic impedance between
the human body and the ultrasonic transducers. One or more acoustic
matching layers are formed for change the difference in the
acoustic impedance stepwise to reduce a loss of the ultrasonic
waves in the propagation. However, JP-A 6-148154 does not suggest
consideration of the difference in the acoustic impedance between
the human body and the ultrasonic transducers.
[0009] The ultrasonic transducers disclosed in JP-A 6-148154 have a
problem of a high manufacturing cost due to a complicated structure
in which a recess is formed in the acoustic lens and the ultrasonic
transducers for reception are disposed in the recess.
SUMMARY OF THE INVENTION
[0010] In view of the foregoing problems, an object of the present
invention is to provide an ultrasonic transducer of which
sensitivity can be raised with a simple structure, and ultrasonic
probe and producing method.
[0011] In order to achieve the above and other objects and
advantages of this invention, an ultrasonic transducer includes a
first piezoelectric layer for transmitting ultrasonic waves to an
object in a body. A second piezoelectric layer receives the
ultrasonic waves reflected by the object. An acoustic matching
layer is disposed between the first and second piezoelectric
layers, for constituting an electrode common to the first and
second piezoelectric layers.
[0012] The second piezoelectric layer is positioned nearer to the
object than the first piezoelectric layer.
[0013] The first piezoelectric layer is inorganic, and the second
piezoelectric layer is organic.
[0014] The first piezoelectric layer is inorganic, and the second
piezoelectric layer is a complex containing an organic compound and
an inorganic compound dispersed in the organic compound.
[0015] The acoustic matching layer contains metal.
[0016] The metal is constituted by silver.
[0017] The acoustic matching layer is a complex containing an
organic compound and metal.
[0018] The organic compound is adhesive.
[0019] The metal is constituted by metal nano particles.
[0020] The acoustic matching layer includes a first layer of a
material with a relatively low conductivity or a non-conductive
material. A conductive material is overlaid on the first layer.
[0021] Furthermore, a second acoustic matching layer is overlaid on
a surface of the second piezoelectric layer opposite to the
acoustic matching layer.
[0022] Each of the first and second piezoelectric layers includes
plural element regions arranged in an array in the first
direction.
[0023] Preferably, the second piezoelectric layer includes plural
receiving element regions, arranged in a first direction, and
having volumes different from one another.
[0024] The plural element regions have receiving surfaces, which
are arranged in the first direction, for receiving the reflected
ultrasonic waves, and of which areas are different from one
another.
[0025] The plural element regions have thicknesses different from
one another in a direction perpendicular to receiving surfaces
thereof.
[0026] The element regions are arranged two-dimensionally in the
first direction and a second direction.
[0027] Also, an ultrasonic probe having an ultrasonic transducer is
provided. The ultrasonic transducer includes a first piezoelectric
layer for transmitting ultrasonic waves to an object in a body. A
second piezoelectric layer receives the ultrasonic waves reflected
by the object. An acoustic matching layer is disposed between the
first and second piezoelectric layers, for constituting an
electrode common to the first and second piezoelectric layers.
[0028] Also, an ultrasonic transducer includes a piezoelectric
layer. Plural acoustic matching layers are overlaid on the
piezoelectric layer. The acoustic matching layers include a first
acoustic matching layer, positioned on the piezoelectric layer,
formed from a mixture of metal nano particles and adhesive resin,
for constituting an electrode for the piezoelectric layer.
[0029] The metal nano particles are constituted by silver nano
particles.
[0030] The acoustic matching layers are at least three acoustic
matching layers of which acoustic impedance is different from one
another.
[0031] Also, an ultrasonic probe having an ultrasonic transducer is
provided. The ultrasonic transducer includes a piezoelectric layer.
Plural acoustic matching layers are overlaid on the piezoelectric
layer. The acoustic matching layers include a first acoustic
matching layer, positioned on the piezoelectric layer, formed from
a mixture of metal nano particles and adhesive resin, for
constituting an electrode for the piezoelectric layer.
[0032] Also, a method of producing an ultrasonic transducer having
a piezoelectric layer is provided, and includes a step of placing a
first acoustic matching layer on the piezoelectric layer, the first
acoustic matching layer containing a mixture of metal nano
particles and adhesive resin, and constituting an electrode for the
piezoelectric layer. A second acoustic matching layer different
from the first acoustic matching layer is placed on the first
acoustic matching layer. The first acoustic matching layer is
hardened by heating, so as to attach the first acoustic matching
layer to the piezoelectric layer and to attach the second acoustic
matching layer to the first acoustic matching layer.
[0033] Consequently, sensitivity of the ultrasonic probe can be
raised with a simple structure, because the acoustic matching layer
is disposed between the first and second piezoelectric layers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] The above objects and advantages of the present invention
will become more apparent from the following detailed description
when read in connection with the accompanying drawings, in
which:
[0035] FIG. 1 is a perspective view illustrating an ultrasonic
diagnostic apparatus;
[0036] FIG. 2 is an explanatory view illustrating a layered
structure of an ultrasonic transducer array;
[0037] FIG. 3 is a block diagram schematically illustrating circuit
elements in the ultrasonic transducer array;
[0038] FIG. 4 is an explanatory view illustrating one preferred
ultrasonic transducer array in which a receiving piezoelectric
layer has receiving element regions;
[0039] FIG. 5 is an explanatory view illustrating another preferred
ultrasonic transducer array having three acoustic matching
layers;
[0040] FIG. 6 is a flow chart illustrating a sequence of
fabricating the ultrasonic transducer array;
[0041] FIG. 7 is an explanatory view illustrating a comparative
structure of an ultrasonic transducer array having two acoustic
matching layers.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE PRESENT
INVENTION
[0042] In FIG. 1, an ultrasonic diagnostic system 2 or apparatus
includes a portable ultrasonic imaging apparatus 10 and an
ultrasonic probe 11. The ultrasonic imaging apparatus 10 or device
includes a housing 12 and a cover 13 or display unit. An input
panel 14 is disposed on an upper surface of the housing 12,
includes plural buttons, trackball and the like, and is operated
for inputting various data. A monitor display panel 15 is secured
to the inside of the cover 13 for displaying various menus,
ultrasonic images, and the like.
[0043] A hinge 16 keeps the cover 13 rotatable on the housing 12.
The cover 13 is rotatable between an open position and a closed
position (not shown), and when in the open position, reveals the
input panel 14 and the display panel 15, and when in the closed
position, covers and protects the panels 14 and 15 by opposing an
inner surface of the cover 13 to an upper surface of the housing
12. A grip (not shown) is secured to a lateral face of the housing
12, and held manually for carrying the ultrasonic imaging apparatus
10 when the cover 13 is in the closed position on the housing 12. A
connecting port 17 is formed in another surface of the housing 12
for connecting the ultrasonic probe 11 removably.
[0044] The ultrasonic probe 11 includes a scan head 18, a connector
plug 19, and a cable 20. The scan head 18 is manually held by a
doctor or operator, and caused to touch a patient's body. The cable
20 extends between the scan head 18 and the connector plug 19. An
ultrasonic transducer array 21 or UT array is incorporated in an
end portion of the scan head 18.
[0045] In FIG. 2, the ultrasonic transducer array 21 includes a
base plate 25 and various layers overlaid thereon. The base plate
25 is formed from glass, epoxy resin or the like. The layers
include backing material 26, a lower electrode 27, a transducer
piezoelectric layer 28, a first acoustic matching layer 29, a
receiving piezoelectric layer 30, an upper electrode 31, a second
acoustic matching layer 32, and an acoustic lens 33 in a listed
order.
[0046] The backing material 26 regulates free vibration of the
transducer piezoelectric layer 28 in the course of emitting
ultrasonic waves, to raise a resolving power of the ultrasonic
waves in an emission direction. Various materials can be used for
the backing material 26 with property to absorb vibration, and can
be organic or inorganic. Preferable examples of the materials
include epoxy resin and other resins, chlorinated polyethylene
rubber, natural rubber, styrene-butadiene rubber (SBR) and other
rubbers, because they have low acoustic impedance and can absorb
vibration without dropping the sensitivity.
[0047] The transducer piezoelectric layer 28 is present in an array
of plural element regions of strips, which extend in the EL
direction. The element regions of the transducer piezoelectric
layer 28 are disposed equidistantly in the array of the AZ
direction perpendicular to the EL direction. Filler 34 is charged
in gaps or clearances between the element regions of the transducer
piezoelectric layer 28 and on their peripheral surface. The array
of the element regions are shaped by either surface patterning or
cutting (dicing).
[0048] The transducer piezoelectric layer 28 has a thickness of
0.1-0.5 mm, and is sandwiched by the first acoustic matching layer
29 and the lower electrode 27. The first acoustic matching layer 29
is electrically conductive, and is used as an upper electrode for
the transducer piezoelectric layer 28. The lower electrode 27 is a
thin film of metal. Lead wires (not shown) are connected with
respectively the first acoustic matching layer 29 and the lower
electrode 27. The lower electrode 27, the transducer piezoelectric
layer 28 and the first acoustic matching layer 29 are combined to
constitute an ultrasonic transducer for transmission and
reception.
[0049] The lead wire connected with the first acoustic matching
layer 29 is grounded as illustrated in FIG. 3. A lead wire
connected with the lower electrode 27 is connected to the
ultrasonic imaging apparatus 10. A transmitter/receiver 41 of FIG.
3 is incorporated in the ultrasonic imaging apparatus 10. When a
pulse voltage is applied to the transducer piezoelectric layer 28
by the transmitter/receiver 41, the transducer piezoelectric layer
28 vibrates to generate ultrasonic waves for emission to an object
of interest in a body cavity. When the transmitter/receiver 41
receives echo or reflected waves from the object, the transducer
piezoelectric layer 28 vibrates to output echo information
(reception signal) as a voltage signal.
[0050] Various inorganic materials with piezoelectric property can
be used for the transducer piezoelectric layer 28. Preferable
examples of the materials are PZT and other Pb compounds with
piezoelectric property. Among those, specifically preferable
examples are PMN-PT and PZN-PT as piezoelectric relaxor single
crystal which can have an ultrahigh piezoelectric coefficient.
Those are characterized in a high value of the electromechanical
coupling coefficient k and in a high value of a ratio of output of
ultrasonic waves to applied voltage as efficiency of
conversion.
[0051] The receiving piezoelectric layer 30 operates also as an
acoustic matching layer, and cooperates with the first and second
acoustic matching layers 29 and 32. A difference in the acoustic
impedance between the transducer piezoelectric layer 28 and a human
body is reduced stepwise to increase sensitivity of transmission
and reception of ultrasonic waves. The second acoustic matching
layer 32 reduces a difference in the acoustic impedance between the
receiving piezoelectric layer 30 and the human body stepwise, to
increase sensitivity of transmission and reception of ultrasonic
waves.
[0052] Various conductive materials can be used for the first
acoustic matching layer 29 with an acoustic impedance lower than
that of the transducer piezoelectric layer 28 and higher than that
of the receiving piezoelectric layer 30. A preferable material for
the first acoustic matching layer 29 is a baked mixture of metal
nano particles (with a diameter of 1-100 nm) and adhesive resin.
Preferable metal nano particles are silver nano particles.
Specifically, the metal nano particles can exclusively consist of
silver nano particles. This is because the silver nano particles
have relatively high dispersibility to resin among various metals.
When resin with the metal nano particles is baked, particles become
bonded in the resin to form a conductive path. Thus, high
conductivity can be obtained. Note that the first acoustic matching
layer 29 may be a dual layer structure, which can include a first
layer of a material without conductivity or a relatively low
conductivity, and a conductive layer overlaid on the first
layer.
[0053] In a manner similar to the transducer piezoelectric layer
28, the receiving piezoelectric layer 30 is constituted by plural
element regions of cutting (dicing), or is present with plural
element regions of strips of surface patterning for splitting
electrically. Filler 35 is charged in gaps of the receiving
piezoelectric layer 30 and on its peripheral surface.
[0054] The receiving piezoelectric layer 30 has a thickness of
0.05-0.3 mm, and sandwiched by the upper electrode 31 and the first
acoustic matching layer 29. The first acoustic matching layer 29
operates as a lower electrode for the receiving piezoelectric layer
30 in addition to operating as upper electrode for the transducer
piezoelectric layer 28 as described above. It follows that the
first acoustic matching layer 29 functions as a common electrode
for the transducer piezoelectric layer 28 and the receiving
piezoelectric layer 30. The upper electrode 31 is a film of metal,
to which a lead wire (not shown) is connected.
[0055] The lead wire connected with the upper electrode 31 is
connected to the ultrasonic imaging apparatus 10. When the
receiving piezoelectric layer 30 receives echo or reflected waves,
echo information is input to the transmitter/receiver 41 in the
ultrasonic imaging apparatus 10 in FIG. 3. A combination of the
first acoustic matching layer 29, the receiving piezoelectric layer
30 and the upper electrode 31 constitutes an ultrasonic transducer
for reception.
[0056] Various organic materials with piezoelectric property can be
used for the receiving piezoelectric layer 30 with an acoustic
impedance lower than that of the first acoustic matching layer 29
and higher than that of the second acoustic matching layer 32.
Among those, preferable materials are PVDF, P(VDF-TrFE) and other
fluorocarbon polymers. Those are characterized in a high
sensitivity factor g, and relatively high sensitivity to ultrasonic
waves. Note that the receiving piezoelectric layer 30 can be formed
from complex piezoelectric material formed by dispersing an
inorganic compound in an organic compound.
[0057] Material for the second acoustic matching layer 32 can be a
compound of which acoustic impedance is lower than that of the
receiving piezoelectric layer 30 and higher than that of a human
body. A preferable example of the material is epoxy resin.
[0058] The acoustic lens 33 converges ultrasonic waves from the
transducer piezoelectric layer 28 at an object of interest in a
body cavity. A material for the acoustic lens 33 is silicone rubber
with a maximum thickness of 1 mm or so.
[0059] Various adhesive materials can be used for adhesion in
overlaying plural layers in the ultrasonic transducer array 21.
Among those, epoxy resin as adhesive agent is specifically
preferable for high acoustic transmittance, high strength in
attachment, and low cost.
[0060] In FIG. 3, the transmitter/receiver 41 includes a pulser 42,
a receiver 43, a first A/D converter 44, an image producing device
45, a transmitting/receiving switch 46 or TX/RX switch, a receiver
47, and a second A/D converter 48.
[0061] The pulser 42 is connected with the lower electrode 27 by
the transmitting/receiving switch 46. The pulser 42 transmits a
pulse voltage or excitation pulse to the lower electrode 27 for the
transducer piezoelectric layer 28 to generate ultrasonic waves.
[0062] The receiver 43 is connected with the lower electrode 27 by
the transmitting/receiving switch 46. The receiver 43 is supplied
with echo information (reception signal) from the transducer
piezoelectric layer 28 according to reflected waves or echo from
the object of interest.
[0063] The first A/D converter 44 converts the echo information
from the receiver 43 into a digital form according to the A/D
conversion. The digital echo data from the first A/D converter 44
is input to the image producing device 45.
[0064] The pulser 42 and the receiver 43 are connected with the
transmitting/receiving switch 46, which changes over inputs and
outputs of those selectively.
[0065] The receiver 47 is connected with the upper electrode 31,
and amplifies the echo information from the receiving piezoelectric
layer 30 in a manner similar to the receiver 43.
[0066] The second A/D converter 48 converts the echo information
from the receiver 47 into a digital form according to the A/D
conversion. The digital echo data from the second A/D converter 48
is input to the image producing device 45. The image producing
device 45 produces an ultrasonic image from the echo information
input by the first and second A/D converters 44 and 48, and outputs
the image to the display panel 15. Various elements included in the
transmitter/receiver 41 except for the image producing device 45
are also incorporated for the transducer piezoelectric layer 28 and
the receiving piezoelectric layer 30.
[0067] Thus, the ultrasonic transducer array can be easily
fabricated in a simple structure without high cost, because the
first acoustic matching layer 29 with electric conductivity is used
as a common electrode for the transducer piezoelectric layer 28 and
the receiving piezoelectric layer 30. The first acoustic matching
layer 29 can have the high conductivity owing to the use of resin
containing silver nano particles, and can be used sufficiently as
common electrode.
[0068] The resin for the first acoustic matching layer 29 is a
thermoset compound and operates as adhesive for the receiving
piezoelectric layer 30. The ultrasonic transducer array 21 can be
fabricated easily by reducing the number of steps in the
fabrication in comparison with a known method in which adhesive
agent is applied to attach the receiving piezoelectric layer 30
after forming the first acoustic matching layer. The number of
electrodes of a relatively small thickness can be kept small, to
prevent drop of the yield of the fabrication.
[0069] In the present embodiment, the transducer piezoelectric
layer 28 is used for transmission and reception. However,
transmission ultrasonic transducer only for transmission without
reception can be used instead of the transducer piezoelectric layer
28.
[0070] If a transmission line path between the upper electrode 31
and the receiver 47 is very long, a drop of voltage of the echo
information between the upper electrode 31 and the receiver 47 will
be considerably great according to line resistance of the
transmission line path. This will cause degradation of an
ultrasonic image according to ultrasonic waves received by the
receiving piezoelectric layer 30, to reduce effect of the substance
of the receiving piezoelectric layer 30 with high sensitivity.
[0071] Thus, the transmission line path for connection between the
upper electrode 31 and the receiver 47 should be as short as
possible. The receiver 47 is preferably disposed very near to the
upper electrode 31. Specifically, the receiver 47 is contained in
the scan head 18 instead of containment in the ultrasonic imaging
apparatus 10.
Example 1
[0072] The ultrasonic transducer array 21 was formed
experimentally. The backing material 26 was chlorinated
polyethylene rubber and had a thickness of 1 cm. A flexible printed
circuit board (FPC) was attached to the backing material 26 by
adhesion of epoxy resin of a thermoset type.
[0073] A film of the transducer piezoelectric layer 28 was formed
from C92H (trade name, manufactured by Fuji Ceramics Corporation)
as piezoelectric ceramic PZT material. The film of the transducer
piezoelectric layer 28 was polished for both its surfaces to have a
thickness of 260 microns. A film of metal was formed on one surface
of the transducer piezoelectric layer 28 by sputtering Ti, Pt and
Au one after another. Then a proximal side of the transducer
piezoelectric layer 28 having the metal film was attached to the
flexible printed circuit board (FPC) on the backing material 26 by
adhesion. To this end, adhesive agent was used, which contained
silver nano particles, and was thermally hardened. Acoustic
impedance of the transducer piezoelectric layer 28 was
approximately 31 Mrayl. Note that the lower electrode 27 was
constituted by the flexible printed circuit board (FPC) on the
backing material 26 and the metal film of Ti, Pt and Au on the
transducer piezoelectric layer 28.
[0074] As the first acoustic matching layer 29, a coating of resin
containing silver nano particles (produced by Sumitomo Denki Kogyo
Co., Ltd.) was applied to the transducer piezoelectric layer 28
with a thickness of .lamda./4 (where .lamda. is a wavelength of
ultrasonic waves). The first acoustic matching layer 29 was
hardened by heating at approximately 180 deg. C. for one (1) hour
in the atmosphere. The acoustic impedance of the first acoustic
matching layer 29 after heating was approximately 12 Mrayl.
[0075] The receiving piezoelectric layer 30 was formed from PVDF
having acoustic impedance of 4.5 Mrayl. The receiving piezoelectric
layer 30 was molded at a thickness of .lamda./4. An surface
electrode or film of metal was formed entirely on one surface of
the receiving piezoelectric layer 30. A second surface of the
receiving piezoelectric layer 30 opposite to the surface electrode
was attached to the first acoustic matching layer 29. For the
attachment, adhesive agent containing silver nano particles was
used. The surface electrode on the receiving piezoelectric layer 30
constituted the upper electrode 31.
[0076] A film of epoxy adhesive agent with acoustic impedance of 2
Mrayl was polished to have a thickness of .lamda./4, and was
attached to the upper electrode 31 as the second acoustic matching
layer 32 by use of epoxy adhesive agent of a thermoset type.
Similarly, the acoustic lens 33 was attached to the second acoustic
matching layer 32.
[0077] For further examples, elements not included in Example 1
will be hereinafter described.
Example 2
[0078] The transducer piezoelectric layer 28 was formed from PMN-PT
(trade name) as a Pb (Mg, Nb) O.sub.3--PbTiO.sub.3 compound
(manufactured by JFE Mineral Corporation) for a piezoelectric
relaxor single crystal. The film of the transducer piezoelectric
layer 28 was polished for both its surfaces to have a thickness of
240 microns. Acoustic impedance of the transducer piezoelectric
layer 28 was approximately 22 Mrayl.
[0079] In a manner similar to Example 1, the first acoustic
matching layer 29 was formed and then hardened by heating at
approximately 160 deg. C. for one (1) hour in the atmosphere. The
acoustic impedance of the first acoustic matching layer 29 after
heating was approximately 11 Mrayl.
Comparative Example 1
[0080] Each of both surfaces of the transducer piezoelectric layer
28 was coated with metals of Ti, Pt and Au successively by
sputtering to form a metal film. After this, the transducer
piezoelectric layer 28 was attached to the flexible printed circuit
board (FPC) positioned on the backing material 26. Adhesive agent
was used for the attachment, was resin containing silver nano
particles. The transducer piezoelectric layer 28 on the flexible
printed circuit board (FPC) was hardened by heating at
approximately 100 deg. C. for one (1) hour in the atmosphere. The
transducer piezoelectric layer 28 had acoustic impedance of
approximately 31 Mrayl.
[0081] An upper electrode was disposed instead of the first
acoustic matching layer 29. A film of metal was formed on the
transducer piezoelectric layer 28 and was the upper electrode. An
acoustic matching layer was formed instead of the receiving
piezoelectric layer 30. Epoxy resin dispersion of zirconia
particles was used, had acoustic impedance of 8 Mrayl, was polished
to have a thickness of .lamda./4, and was overlaid on the
transducer piezoelectric layer 28 as the acoustic matching layer.
Epoxy adhesive agent of a thermoset type was used, and hardened by
heating.
[0082] A film of epoxy adhesive agent with acoustic impedance of 3
Mrayl was polished to have a thickness of .lamda./4, and was
attached as the second acoustic matching layer 32 by use of the
epoxy adhesive agent of the thermoset type.
TABLE-US-00001 TABLE 1 Comparative Example 1 Example 2 example 1
Transducer piezo- piezo- piezo- piezo- electric electric electric
electric layer ceramic PZT, relaxor single ceramic PZT, 28 31 Mrayl
crystal, 31 Mrayl 22 Mrayl First acoustic resin resin x matching
layer containing containing 29 silver nano silver nano particles,
particles, 12 Mrayl 11 Mrayl Receiving PVDF, PVDF, epoxy resin
piezo- 4.5 Mrayl 4.5 Mrayl dispersion of electric layer zirconia 30
particles, 8 Mrayl Second epoxy resin, epoxy resin, epoxy resin,
acoustic 2 Mrayl 2 Mrayl 3 Mrayl matching layer 32
[0083] Table 1 above indicated a relationship between materials for
the layers in Examples and Comparative examples, and values of the
acoustic impedance (Mrayl). The transducer piezoelectric layer 28
was formed from the piezoelectric relaxor single crystal and had
acoustic impedance of 22 Mrayl only according to Example 2, but was
formed from the piezoelectric ceramic PZT and had acoustic
impedance of 31 Mrayl according to the remaining examples.
[0084] The first acoustic matching layer 29 of Examples 1 and 2 was
formed from resin containing silver nano particles. The acoustic
impedance of the first acoustic matching layer 29 was 12 Mrayl in
Example 1, and was 11 Mrayl in Example 2. In contrast, the first
acoustic matching layer 29 was not present in Comparative example
1.
[0085] The receiving piezoelectric layer 30 of Examples 1 and 2 was
formed from PVDF with the acoustic impedance of 4.5 Mrayl. In
contrast, the resin layer of Comparative example 1 instead of the
receiving piezoelectric layer 30 was formed from epoxy resin
dispersion of zirconia particles with the acoustic impedance of 8
Mrayl.
[0086] The second acoustic matching layer 32 in any of the examples
was formed from the epoxy resin. The acoustic impedance of the
second acoustic matching layer 32 was 3 Mrayl in Example 1, and was
2 Mrayl in all the remaining examples.
[0087] An experiment was conducted to test sensitivity of reception
of the ultrasonic transducer array 21 fabricated according to
Examples 1 and 2 and Comparative example 1. An ultrasonic wave
comes to have a distorted waveform in the course of propagation,
and comes to include higher harmonic components with a frequency an
integer times the frequency of the fundamental wave. As a result,
higher harmonic components were received with high sensitivity in
addition to a fundamental wave in any of Examples 1 and 2 and
Comparative example 1.
[0088] In Comparative Example 1, in contrast, the fundamental wave
was received when an output frequency was equal to or more than a
predetermined frequency. However, it was impossible to receive
second harmonic waves. Note that it was possible to receive second
harmonic waves by lowering the output frequency, but the lowered
output frequency was insufficient for the ultrasonic imaging.
[0089] Thus, it was possible with a high sensitivity to receive
second harmonic waves in addition to the fundamental wave owing to
the receiving piezoelectric layer 30 separately in combination with
the transducer piezoelectric layer 28.
[0090] In the above embodiments, the ultrasonic probe is a type of
convex scan for extracorporeal use. However, a ultrasonic probe of
the invention may be a type of a radial scan or a type of a
mechanical scan for mechanically rotating, swinging or sliding a
single ultrasonic transducer. The feature of the invention can be
used in a ultrasonic probe insertable in an instrument channel in
an electronic endoscope for in-vivo use, an ultrasonic endoscope
structured integrally with an electronic endoscope.
[0091] In FIG. 4, another preferred structure of the ultrasonic
transducer array 21 is illustrated. The transducer piezoelectric
layer 28 in the ultrasonic transducer array 21 includes transducer
element regions 28a of the array. The receiving piezoelectric layer
30 includes receiving element regions 30a-30e. The transducer
piezoelectric layer 28 and the receiving piezoelectric layer 30 are
combined with the base plate 25, the backing material 26, the lower
electrode 27, the first acoustic matching layer 29, the upper
electrode 31, the second acoustic matching layer 32 and the
acoustic lens 33.
[0092] The receiving element regions 30a-30e are arranged
equidistantly in the AZ direction, and have a two-dimensional
multi-element form. The receiving element regions 30a-30e have
receiving surfaces for reflected wave with areas decreasing in the
EL direction along the receiving piezoelectric layer 30, and have
volumes different from one another. Each of the receiving element
regions 30a-30e has a specific frequency derived from the volume,
shape and the like to receive reflected waves with high sensitivity
for specific frequency according to the specific frequency.
[0093] Waveforms of the ultrasonic waves become distorted by
influence of physical property (hardness and the like) of tissue of
a body cavity, and will include harmonic components (non linear
components) as an integer times the frequency of the fundamental
wave. An area, shape and the like of a receiving surface of the
receiving element regions 30a-30e for reflected waves along the
receiving piezoelectric layer 30 are predetermined for a specific
frequency according to frequencies of the fundamental wave and
harmonic waves. This is effective in receiving reflected waves with
high sensitivity, especially harmonic components, such as a second
harmonic wave, third harmonic wave, fourth harmonic wave and the
like.
[0094] Also, a non-linear parameter (B/A parameter) can be obtained
from a ratio between the received fundamental wave and higher
harmonic components as information of possibility of a non linear
phenomenon on the tissue in a body cavity. It has been expected to
utilize the B/A parameter as diagnostic value of next generation
because of correspondence to hardness of the tissue. In the present
invention, the B/A parameter can be obtained reliably and easily
owing to exact examination.
[0095] In the present embodiment, areas of the receiving element
regions 30a-30e as receiving surfaces for reflected waves arranged
along the receiving piezoelectric layer 30 decreases in the EL
direction from each of the two ends toward the center. However,
these areas of the receiving element regions 30a-30e can be
determined with changes in other manners, for example, can decrease
in a direction from the center toward each of the two ends. Also,
instead of or in addition to a decrease in the areas of the
receiving surfaces of the receiving element regions 30a-30e, it is
possible to set the thicknesses different between those in a
direction vertical to the receiving surfaces for the reflected
waves of the receiving piezoelectric layer 30. When the thickness
is reduced to 1/n time as much as the initial thickness (wherein n
is an integer), a specific frequency becomes n times as high as
before. For example, when the thickness is reduced to a half as
much as the initial thickness, the specific frequency becomes two
times as high as before.
Example 3
[0096] An experiment was conducted for the embodiment. In Example
3, the ultrasonic transducer array 21 of Example 1 was repeated
with a difference in using the structure of FIG. 4.
Example 4
[0097] Example 2 was repeated with a difference in using the
structure of FIG. 4.
Comparative Example 2
[0098] Comparative example 1 was repeated for comparison.
[0099] The experiment was conducted to test sensitivity of
reception of the ultrasonic transducer array 21 fabricated
according to Examples 3 and 4 and Comparative example 2. As a
result, higher harmonic components were received with high
sensitivity in addition to a fundamental wave in any of Examples 3
and 4.
[0100] In Comparative Example 2, in contrast, the fundamental wave
was received when the output frequency was equal to or more than a
predetermined frequency. However, it was impossible to receive a
higher harmonic component. Note that it was possible to receive a
higher harmonic component by lowering the output frequency, but the
lowered output frequency was insufficient for the ultrasonic
imaging.
[0101] Thus, it was possible to receive a higher harmonic component
in addition to the fundamental wave owing to the receiving element
regions 30a-30e with the different volumes in combination with the
transducer element regions 28a.
[0102] In FIG. 7, a known ultrasonic transducer array 101 or UT
array is illustrated in a manner of JP-A 9-139998. Plural
ultrasonic transducers 102 of the array are arranged in the AZ
direction. The ultrasonic transducers 102 have a piezoelectric
layer 103 and electrodes 104 and 105 for applying voltage to the
piezoelectric layer 103. Lead wires (not shown) are connected with
the electrodes 104 and 105. A base plate 107 and backing material
106 are disposed on a surface of the ultrasonic transducers 102
opposite to an emitting surface for emitting ultrasonic waves. The
backing material 106 absorbs unwanted ultrasonic waves from the
ultrasonic transducers 102.
[0103] There is a considerable difference in the acoustic impedance
between the ultrasonic transducers 102 and tissue of a body cavity.
That of the tissue is approximately 1.5 Mrayl, in contrast with
approximately 25-35 Mrayl as that of piezoelectric ceramic material
in the ultrasonic transducers 102. Due to the difference,
ultrasonic waves are reflected by an interface between the
ultrasonic transducers 102 and the tissue to cause a loss in the
propagation. In view of this, a first acoustic matching layer 108
and a second acoustic matching layer 109 are formed on the
ultrasonic transducers 102 on their emission surface. That is
changed stepwise from the ultrasonic transducers 102 to the tissue.
Note that an acoustic lens 110 operates to converge ultrasonic
waves in the EL direction which is perpendicular to the AZ
direction of the array.
[0104] A difference in the acoustic impedance is reduced stepwise
by the multi layer structure with plural acoustic matching layers.
Theoretically, ultrasonic reflection decreases on an interface of
the ultrasonic transducer and tissue of an object of interest, to
prevent a loss in the propagation of ultrasonic waves to raise
sensitivity in receiving and transmitting ultrasonic waves.
However, a total of the interfaces between the acoustic matching
layers will considerably large. Adhesive agent present on the
interfaces reflects the ultrasonic waves so as to lower the
sensitivity of reception and transmission. Another problem arises
in that the plural acoustic matching layers must be attached to one
another, so that the yield of the fabrication will be small and the
process time will be long due to their small thickness.
[0105] If the acoustic matching layer is in the multi layer
structure, a first one of acoustic matching layers on an emitting
surface of the ultrasonic transducer should satisfy a condition
with an impedance which is lower than that of piezoelectric ceramic
material and higher than that of organic substances for forming a
second one of the acoustic matching layers on the object side
(distal side). However, known materials satisfying the condition
are very few. A range of selecting the materials is considerably
limited.
[0106] An embodiment to solve the problem is illustrated in FIG. 5.
The ultrasonic transducer array 21 includes a first acoustic
matching layer 49, a second acoustic matching layer 50, a third
acoustic matching layer 51 and an acoustic lens 52 in combination
with the base plate 25, the backing material 26, the lower
electrode 27 and the transducer piezoelectric layer 28.
[0107] Various conductive materials can be used for the first
acoustic matching layer 49 with an acoustic impedance lower than
that of the transducer piezoelectric layer 28 and higher than that
of the second acoustic matching layer 50.
TABLE-US-00002 TABLE 2 Baking temperature (deg. C.) 100 150 200
Sonic speed 1,560 1,820 2,080 (m/s) Density (kg/m.sup.3) 3,910
5,560 7,210 Acoustic 6.1 10.1 15.0 impedance (Mrayl)
[0108] Table 2 indicates the sonic speed, density and acoustic
impedance of the acoustic matching layer containing silver nano
particles as metal nano particles, for each value of the baking
temperature. According to the increase in the baking temperature
(deg. C.) from 100 and 150 to 200, the sonic speed (m/s) increases
in a sequence of 1,560, 1,820 and 2,080. Similarly, the density
(kg/m.sup.3) increases in a sequence of 3,910, 5,560 and 7,210. The
acoustic impedance (Mrayl) increases in a sequence of 6.1, 10.1 and
15.0.
[0109] It follows that baking at a high temperature refines the
acoustic matching layer to enlarge acoustic impedance. It is
possible to determine acoustic impedance at suitable levels by
changing the baking temperature in the acoustic matching layer
containing silver nano particles. The acoustic matching layer can
be used suitably in many types of piezoelectric layers. It is
unnecessary to select materials for forming the acoustic matching
layer with a desired acoustic impedance, to minimize labor for
manual operation.
[0110] Various materials can be used for the second acoustic
matching layer 50 with an acoustic impedance lower than that of the
first acoustic matching layer 49 and higher than that of the third
acoustic matching layer 51. Among those, a preferable material is a
mixture of zirconia particles and epoxy resin. Various materials
can be used for the third acoustic matching layer 51 with an
acoustic impedance lower than that of the second acoustic matching
layer 50 and higher than that of a human body. Among those, a
preferable material is epoxy resin.
[0111] A sequence of fabricating the ultrasonic transducer array 21
is described by referring to FIG. 6. At first, the lower electrode
27 and the transducer piezoelectric layer 28 are overlaid on the
backing material 26, and cut in elements of plural strips by dicing
in the step S10. The filler 34 is filled in gaps between the
elements of the transducer piezoelectric layer 28 and around those.
Then an upper surface of the transducer piezoelectric layer 28 is
coated with a mixed material as the first acoustic matching layer
49 inclusive of metal nano particles and adhesive resin. See the
step S11. Then a mixed material is caused to flow and shaped in a
film form, inclusive of zirconia particles and epoxy resin, so then
the second acoustic matching layer 50 is overlaid on the first
acoustic matching layer 49 of the transducer piezoelectric layer
28. See the step S12. The layers are heated at the step S13. The
first acoustic matching layer 49 is hardened by heat to attach the
first and second acoustic matching layers 49 and 50 to the
transducer piezoelectric layer 28. Also, the third acoustic
matching layer 51 and the acoustic lens 52 are overlaid in the step
S14 to obtain the ultrasonic transducer array 21. Note that the
step of dicing the lower electrode 27 and the transducer
piezoelectric layer 28 into the array can be performed at one time
for the acoustic matching layers 49-51 after overlaying all the
three. In a manner similar to the steps S10-S14, filler is charged
in the gaps after the dicing.
[0112] As the first acoustic matching layer 49 operates as adhesive
to overlay the second acoustic matching layer 50 thereon, the
ultrasonic transducer array 21 can be fabricated easily by reducing
the number of steps in the fabrication in comparison with a known
method in which adhesive agent is applied to form the second
acoustic matching layer 50 after forming the first acoustic
matching layer. Drop of the yield of the fabrication can be
prevented.
[0113] In the present embodiment, the acoustic matching layers
49-51 are present. However, the acoustic matching layers can be two
or four or more layers which can include one formed from a mixture
of metal nano particles and adhesive resin. An increase in the
number of the acoustic matching layers can reduce a difference in
the acoustic impedance between those. It is possible to reduce a
loss in the propagation owing to a decrease in the reflection in
the ultrasonic waves on interfaces between the acoustic matching
layers.
Example 5
[0114] An experiment was conducted for the embodiment. In Example
5, the ultrasonic transducer array 21 of Example 1 was repeated
with a difference in using the structure of FIG. 5.
[0115] For the first acoustic matching layer 49, the transducer
piezoelectric layer 28 was coated with the resin containing silver
nano particles at a thickness of .lamda./4, where .lamda. is a
wavelength of ultrasonic waves. For the second acoustic matching
layer 50, a film of epoxy resin dispersion of zirconia particles
with acoustic impedance of 4 Mrayl was polished to have a thickness
of .lamda./4, and was overlaid on the first acoustic matching layer
49. After this, the second acoustic matching layer 50 was hardened
by heating for one (1) hour at approximately 190 deg. C. in the
atmosphere. The first acoustic matching layer 49 had acoustic
impedance of approximately 14 Mrayl.
Example 6
[0116] Example 2 was repeated with a difference in using the
structure of FIG. 5.
Comparative Example 3
[0117] Comparative example 1 was repeated with the following
differences.
[0118] The first acoustic matching layer 49 was not present.
Instead, a film of metal was formed on the transducer piezoelectric
layer 28 and was an upper electrode.
[0119] For the second acoustic matching layer 50, epoxy resin
dispersion of zirconia particles was used, had acoustic impedance
of 8 Mrayl, was polished to have a thickness of .lamda./4, and was
overlaid on the transducer piezoelectric layer 28.
[0120] For the third acoustic matching layer 51, a film of epoxy
resin was used, had acoustic impedance of 3 Mrayl, was polished to
have a thickness of .lamda./4, and was overlaid on the second
acoustic matching layer 50 by use of epoxy adhesive agent of a
thermoset type.
Comparative Example 4
[0121] A film of metal was formed on each of both surfaces of the
transducer piezoelectric layer 28 by sputtering Ti, Pt and Au one
after another. Then a proximal side of the transducer piezoelectric
layer 28 having the metal film was attached to the flexible printed
circuit board (FPC) on the backing material 26 by adhesion. To this
end, adhesive agent was used, which contained silver nano
particles, and was thermally hardened by heating at approximately
100 deg. C. for one (1) hour in the atmosphere. Acoustic impedance
of the transducer piezoelectric layer 28 was approximately 31
Mrayl.
[0122] In Comparative example 4, an upper electrode was disposed
instead of the first acoustic matching layer 49. A film of metal
was formed on the transducer piezoelectric layer 28 and was the
upper electrode. On the electrode, an acoustic matching layer of
glass was overlaid. The acoustic matching layer of glass had
acoustic impedance of 14 Mrayl, was polished to have a thickness of
.lamda./4, and was overlaid on the transducer piezoelectric layer
28. Epoxy adhesive agent of a thermoset type was used, and hardened
by heating. In general, the acoustic matching layers are diced for
shaping as required for the purpose. It was difficult in the
acoustic matching layer of glass to dice, due to low working
precision according to slipping of a blade for dicing. Also,
polishing was very difficult for the acoustic matching layer of
glass, to lower the yield of fabrication.
[0123] For the second acoustic matching layer 50, epoxy resin
dispersion of zirconia particles was used, had acoustic impedance
of 4 Mrayl, was polished to have a thickness of .lamda./4, and was
overlaid on an acoustic matching layer of glass. Epoxy adhesive
agent of a thermoset type was used, and hardened by heating.
TABLE-US-00003 TABLE 3 Comparative Comparative Example 5 Example 6
example 3 example 4 Transducer piezo- piezo- piezo- piezo- piezo-
electric electric electric electric electric ceramic relaxor
ceramic ceramic layer 28 PZT, single PZT, PZT, 31 Mrayl crystal, 31
Mrayl 31 Mrayl 22 Mrayl First resin resin x glass, acoustic
containing containing 14 Mrayl matching silver nano silver nano
layer 49 particles, particles, 14 Mrayl 11 Mrayl Second epoxy resin
epoxy resin epoxy resin epoxy resin acoustic dispersion dispersion
dispersion dispersion matching of zirconia of zirconia of zirconia
of zirconia layer 50 particles, particles, particles, particles, 4
Mrayl 4 Mrayl 8 Mrayl 4 Mrayl Third epoxy epoxy epoxy epoxy
acoustic resin, resin, resin, resin, matching 2 Mrayl 2 Mrayl 3
Mrayl 2 Mrayl layer 51
[0124] Table 3 above indicated a relationship between materials for
layers in Examples 5 and 6 and Comparative examples 3 and 4, and
values of acoustic impedances (Mrayl).
[0125] The first acoustic matching layer 49 of Examples 5 and 6 was
formed from resin containing silver nano particles. The acoustic
impedance of the first acoustic matching layer 49 was 14 Mrayl in
Example 5, and was 11 Mrayl in Example 6. In contrast, the first
acoustic matching layer 49 was not present in Comparative example
3. In Comparative example 4, the acoustic matching layer of glass
was present instead of the first acoustic matching layer 49, and
had acoustic impedance of 14 Mrayl.
[0126] The second acoustic matching layer 50 in any of Examples and
Comparative examples was formed from epoxy resin dispersion of
zirconia particles. The acoustic impedance of the second acoustic
matching layer 50 was 8 Mrayl in Comparative example 3, and was 4
Mrayl in the remaining Examples and the remaining Comparative
example. The third acoustic matching layer 51 in any of Examples
and Comparative examples was formed from epoxy resin. The acoustic
impedance of the third acoustic matching layer 51 was 3 Mrayl in
Comparative example 3, and was 2 Mrayl in the remaining Examples
and the remaining Comparative example.
[0127] Results of measurement of the sensitivity (dB) and a
fractional bandwidth (%) of the ultrasonic transducer array 21
fabricated according to Examples 5 and 6 and Comparative examples 3
and 4 were indicated in Table 4. The sensitivity was a value
representing a ratio of the voltage between the input and output,
and was higher according to nearness to zero (0). The fractional
bandwidth was a value 100 times a ratio obtained by dividing the
band width (Hz) by the center frequency for the sensitivity of -0.6
dB (half of the ratio of the voltage between the input and output).
The center frequency was approximately 5 MHz for any of Examples 5
and 6 and Comparative examples 3 and 4.
TABLE-US-00004 TABLE 4 Fractional Sensitivity (dB) bandwidth (%)
Example 5 -37.7 69.2 Example 6 -35.5 70.5 Comparative example 3
-41.0 68.7 Comparative example 4 -38.6 65.3
[0128] As a result, the sensitivity of Example 5 was still higher
than that of Comparative example 3 in spite of nearness in the
fractional bandwidth. Note that the number of the acoustic matching
layers in Example 5 was two similar to Comparative example 3 on the
condition that the first acoustic matching layer 49 was not counted
as an acoustic matching layer. Thus, it was possible according to
Example 5 to fabricate the ultrasonic transducer array 21 with very
high sensitivity even with the same number of steps in the
fabrication as Comparative example 3.
[0129] It was found in Example 5 that the sensitivity was higher by
1 dB than Comparative example 4, under the condition in that the
number of the acoustic matching layers in Example 5 was three
similar to Comparative example 4 while the first acoustic matching
layer 49 was counted as an acoustic matching layer. This was
because an adhesive layer was formed to overlay the acoustic
matching layer of glass and the second acoustic matching layer 50,
and results in unacceptable effects. In Example 5, however, no
adhesive was used to overlay the first and second acoustic matching
layers 49 and 50.
[0130] It was found in Example 6 that the sensitivity was higher by
2 dB than Example 5. This was because the piezoelectric relaxor
single crystal was used with the lower acoustic impedance than the
piezoelectric ceramic PZT used in Example 5.
[0131] It was possible to construct a multi layer structure for an
acoustic matching layer easily owing to the first acoustic matching
layer 49 operating also as an upper electrode. Sensitivity was
higher than a known transducer in view of the same number of
acoustic matching layers.
[0132] Although the present invention has been fully described by
way of the preferred embodiments thereof with reference to the
accompanying drawings, various changes and modifications will be
apparent to those having skill in this field. Therefore, unless
otherwise these changes and modifications depart from the scope of
the present invention, they should be construed as included
therein.
* * * * *